The long term objective is to identify and eventually to isolate membrane proteins involved in the trapnsport of glucose, amino acids and other compounds. The immediate goal is to label differentially membrane proteins associated with the transport of glucose amino acids and the Na ion -K ion pump in human rbc and Ehrlich ascites tumor cells. The differential labeling is obtained by using the effect of substrate or reversible inhibitors on the labelling with irreversible inhibitors such as fluorodinitrobenzene.